1. Field of the Invention
This invention relates to hydraulic pumps, and in particular to a variable speed hydraulic pump.
2. Discussion of the Prior Art
Hydraulic pumps are useful for providing power to a work producing device by means of hydraulic fluid under pressure. Hydraulic pumps are used to supply hydraulic fluid pressure for lifting, pressing, punching, and other mechanical operations when used with suitable hydraulic presses, punches, cylinders, and other devices.
Pumps which provide the fluid for these applications typically have a nonlinear flow versus pressure characteristic curve. At low pressures, the flow is high and as the pressure increases, at a certain pressure the flow is drastically reduced. Having a high flow at low pressures greatly reduces cycle times for improved productivity and produces high performance for industrial applications, and the ability to produce high pressures, albeit at lower flows, makes the pump suitable for high force applications.
Pumps of this type are typically a two stage design, utilizing a first stage gear pump and a second stage piston pump. The low pressure pump is either a gear pump, gerotor pump, or a large piston pump. The second stage pump is usually a relatively small diameter piston pump capable of producing high pressures. Below 1000 psi, the first stage pump supplies the oil at a high flow rate. When the pressure reaches about 1000 psi and above, the first stage bypass valve opens to relieve pressure from the first stage pump to the tank pressure, and the second stage pump will supply the fluid at these higher pressures.
The flow of the first stage excess output (the flow not delivered to the load) over the bypass valve creates heat and, in excess, breaks down the oil. Heat exchangers were often required on such pumps to preserve the hydraulic fluid quality. When the second stage pump reaches the maximum pressure, typically around 10,000 psi, the flow from the second stage pump is dumped over a relief valve to limit the pressure. This dumping also creates a large amount of heat because the heat generated is a function of the flow and pressure. These flow characteristics are illustrated in FIG. 7 as the current (prior art) pump. In one aspect, the present invention addresses the problem of excess heat developed in the fluid by dumping fluid over the pressure relief valve at the pressure limit of the pump.
A two stage design is used because such pumps are typically driven by a constant speed electrical motor operating in an open loop mode. An example of a pump having all of these characteristics is the prior art Enerpac 20-Series electric pump, available from Enerpac, a unit of Actuant Corporation, Milwaukee, Wis.
Attempts have been made to make a pump serve low pressures and high pressures with a single pump by varying the speed of the motor which drives the pump. Such attempts have involved measuring the pressure output of the pump, and using that as an input to the motor controller to set the speed of the pump. Requiring a pressure detector adds expense to the pump, making it impractical for many applications.
In a related application, variable displacement axial piston pumps are also currently available. The axial pistons run on a swashplate. The swashplate is hinged to allow the pistons to change their displacement in the piston bores. When the swashplate is at a large angle from 90° to the pistons, the pistons have long strokes and therefore large displacements. When the swashplate is at a small angle from 90°, the pistons have short strokes and therefore small displacements. When the swashplate is at 90° to the pistons the pistons do not stroke and no flow is produced. To make this pump pressure compensated, a piston is attached to the swashplate that senses system pressure. This pump will provide a near constant horsepower system. These pumps are known in the industry and are similar to Rexroth A10VSO. These pumps are generally limited to lower pressures because of the frictional forces that are applied to the swashplate at high pressures.
Oftentimes, hydraulic pumps are used to power single acting hydraulic cylinders. Such cylinders are connected to a single hydraulic line, which provides fluid under pressure to extend or retract the cylinder, and the cylinder is moved in the other direction by a spring when the pressure is relieved. If the hydraulic line is long, or in very cold temperatures in which the hydraulic fluid becomes viscous, the spring may not be strong enough to return the cylinder. In such cases, one method of returning the cylinder is to apply suction to the fluid in the hydraulic line connected to the cylinder. It is an object of the present invention to provide a pump adapted for this as well.